[OC-11769] Make oc_chef_authz a tunable in chef-server.rb (CS12)

During customer loadtesting we found oc_chef_authz http_init_count and http_max_count needed to be tunable. These are not understood by the private_chef helper, so I'm fixing that.

Same fix as https://github.com/opscode/opscode-omnibus/pull/561 but for CS12

CC: @oferrigni @ryancragun